• The role of tundra vegetation in the Arctic water cycle

      Clark, Jason A.; Tape, Ken; Schnabel, William; Euskirchen, Eugénie; Ruess, Roger (2019-12)
      Vegetation plays many roles in Arctic ecosystems, and the role of vegetation in linking the terrestrial system to the atmosphere through evapotranspiration is likely important. Through the acquisition and use of water, vegetation cycles water back to the atmosphere and modifies the local environment. Evapotranspiration is the collective term used to describe the transfer of water from vascular plants (transpiration) and non-vascular plants and surfaces (evaporation) to the atmosphere. Evapotranspiration is known to return large portions of the annual precipitation back to the atmosphere, and it is thus a major component of the terrestrial Arctic hydrologic budget. However, the relative contributions of dominant Arctic vegetation types to total evapotranspiration is unknown. This dissertation addresses the role of vegetation in the tundra water cycle in three chapters: (1) woody shrub stem water content and storage, (2) woody shrub transpiration, and (3) partitioning ecosystem evapotranspiration into major vegetation components. In Chapter 1 I present a method to continuously monitor Arctic shrub water content. The water content of three species (Salix alaxensis, Salix pulchra, Betula nana) was measured over two years to quantify seasonal patterns of stem water content. I found that spring uptake of snowmelt water and stem water storage was minimal relative to the precipitation and evapotranspiration water fluxes. In Chapter 2, I focused on water fluxes by measuring shrub transpiration at two contrasting sites in the arctic tundra of northern Alaska to provide a fundamental understanding of water and energy fluxes. The two sites contrasted moist acidic shrub tundra with a riparian tall shrub community having greater shrub density and biomass. The much greater total shrub transpiration at the riparian site reflected the 12-fold difference in leaf area between the sites. I developed a statistical model using vapor pressure deficit, net radiation, and leaf area, which explained >80% of the variation in hourly shrub transpiration. Transpiration was approximately 10% of summer evapotranspiration in the tundra shrub community and a possible majority of summer evapotranspiration in the riparian shrub community. At the tundra shrub site, the other plant species in that watershed apparently accounted for a much larger proportion of evapotranspiration than the measured shrubs. In Chapter 3, I therefore measured partitioned evapotranspiration from dominant vegetation types in a small Arctic watershed. I used weighing micro-lysimeters to isolate evapotranspiration contributions from moss, sedge tussocks, and mixed vascular plant assemblages. I found that mosses and sedge tussocks are the major constituents of overall evapotranspiration, with the mixed vascular plants making up a minor component. The potential shrub transpiration contribution to overall evapotranspiration covers a huge range and depends on leaf area. Predicted increases in shrub abundance and biomass due to climate change are likely to alter components of the Arctic hydrologic budget. The thermal and hydraulic properties of the moss and organic layer regulate energy fluxes, permafrost stability, and future hydrologic function in the Arctic tundra. Shifts in the composition and cover of mosses and vascular plants will not only alter tundra evapotranspiration dynamics, but will also affect the significant role that mosses, their thick organic layers, and vascular plants play in the thermodynamics of Arctic soils and in the resilience of permafrost.
    • Seedling recruitment, genetic diversity, and secondary growth of deciduous shrubs in Arctic tundra disturbed by retrogressive thaw slump thermokarst on Alaska's North Slope

      Huebner, Diane C.; Bret-Harte, M. Syndonia; Wagner, Diane; Wolf, Diana E.; Douhovnikoff, Vladimir (2020-05)
      Since the 1970s, Arctic temperatures have risen by 2.7 °C, more than twice that of lower latitudes. Productivity of tundra vegetation is historically nutrient-limited, largely due to low rates of decomposition in soils underlain by permafrost, where cold temperatures limit nutrient uptake by plants. However, climate warming is implicated in the recent expansion of tall (≥ 0.5 m) deciduous woody shrubs across the Arctic. Among the largest tundra plants, deciduous shrubs exert strong controls on hydrology, heat balance, nutrient cycling, and food webs. These shrubs may be key players in carbon storage and re-stabilization of thaw-deformed permafrost landscapes (thermokarst), however, shrub-climate feedbacks are complex and their magnitude remains uncertain. Warming associated with recent thermokarst activity includes large (≥ 1 ha) de-vegetated depressions on hillslopes caused by mass soil thaw, known as retrogressive thaw slumps (RTS). RTS have increased on Alaska's North Slope by two-thirds since the 1980s. Within a few decades, some RTS near Toolik Lake support tall willow (Salix spp.) and dwarf birch (Betula nana) colonies. This study quantified three aspects of plant response in RTS of different ages (chronosequences) at two North Slope lakes: 1) recruitment (seedlings m⁻² and percent germination of soil seedbanks), 2) clonal (asexual) growth of dominant vegetation (willow), and 3) secondary growth (annual rings) of dwarf birch and willow. I hypothesized that conditions in RTS support greater recruitment, genetic diversity, and growth than conditions in undisturbed moist acidic tussock tundra, and that the climate signal (June mean temperature) is amplified in RTS shrub ring widths. The study found higher seedling density and seedbank viability associated with warm, nutrient-rich bare soil in recent RTS. Willow species richness was higher in RTS than in undisturbed tundra, but all willows showed high heterozygosity and low clonal spread regardless of disturbance. Ramets (branches) within clones were more widely spaced in RTS, suggesting that RTS can fragment and disperse asexual propagules. Shrub rings in RTS were wider than in undisturbed tundra, but climate sensitivity to warmer temperatures was not amplified in the growth rings of most RTS shrubs. Most RTS shrubs had wider rings associated with greater September precipitation in the previous year, while shrubs growing outside of RTS did not, which suggests protective effects of early snow accumulations in RTS depressions. These results demonstrate that some North Slope RTS support greater seedling recruitment and shrub growth than undisturbed tundra and may enhance tundra shrub growth.